The present invention relates to a new and distinct variety of apple tree named ‘Co-op 31’. Our new tree resulted from a planned hybridization program and is a selection from crossing Rock 41-12 (unpatented) as the seed parent with PRI 841-103 (unpatented) as the pollen parent. The original cross was performed in 1968 in Illinois. The resulting tree was selected when growing in a cultivated area at Urbana, Ill.